1. Field of the Invention
The present invention relates to ultrasonic distance sensors and is concerned more particularly, but not exclusively, with the use of such ultrasonic distance sensors in lithographic apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include, steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In maskless lithography it is known to project the patterned beam onto a target portion of the substrate using a projection system that comprises an array of lenses arranged such that each lens receives and focuses a respective portion of the patterned beam. Each lens of the array of lenses thus projects a respective spot of radiation onto the substrate, and the array of lenses collectively projects a radiation pattern on the substrate. In one type of lithography machine, imaging of the resist layer on the substrate is affected using pixel grid imaging. To achieve this, a large number of small optical spots are imaged onto the resist layer using a series of micro-lens arrays (MLA). The z position of each MLA or group of MLA's is adjustable in order to focus the spots on the surface of the substrate. The focusing of the MLA's is adjustable, so as to compensate for differences in height of the substrate surface. Such focusing adjustment of the MLA's can be effected on the basis of the output of level sensors provided in the vicinity of the substrate surface in a feed forward mode. The effectiveness of conventional systems is limited by an accuracy of such distance sensors.
Therefore, what is needed is a system and method using a more effective distance sensor.